36

Summary

Because of a bug in WP Core, sending multipart emails (html/text) with wp_mail() (to reduce chance of emails ending up in spam folders) will ironically result with your domain being blocked by Hotmail (and other Microsoft emails).

This is a complex problem that I'll aim to break down in great detail in an attempt to help someone find a workable solution which may eventually be implemented in core.

It's going to be a rewarding read. Let's begin...

The bug

The most common advice to avoid having your newsletter emails ending up in spam folders is to send multipart messages.

Multi-part (mime) refers to sending both an HTML and TEXT part of an email message in a single email. When a client receives a multipart message, it accepts the HTML version if it can render HTML, otherwise it presents the plain text version.

This is proven to work. When sending to gmail, all our emails landed in spam folders until we changed the messages to multipart when they came through to main inbox. Great stuff.

Now, when sending multipart messages via wp_mail(), it outputs the Content Type (multipart/*) twice, once with boundary (if customly set) and once without. This behaviour results with the email being displayed as a raw message and not multipart on some emails, including all Microsoft (Hotmail, Outlook, etc...)

Microsoft will flag this message as junk, and the few messages that comes through will be flagged manually by the recipient. Unfortunately, Microsoft emails addresses are widely used. 40% of our subscribers use it.

This is confirmed by Microsoft via an email exchange we had recently.

The flagging of the messages will result with the domain being completely blocked. This means that message will not be sent to spam folder, they will not even be delivered to the recipient at all.

We have had our main domain blocked 3 times so far.

Because this is a bug in the WP core, every domain that sends multipart messages are being blocked. The problem is that most webmasters do not know why. I have confirmed this when doing my research and seeing other users discussing this on forums etc. It requires delving into the raw code and having a good knowledge of how these type of email messages work, which we are going on to next...

Let's break it down into code

Create a hotmail/outlook account. Then, run the following code:

// Set $to to an hotmail.com or outlook.com email
$to = "YourEmail@hotmail.com";

$subject = 'wp_mail testing multipart';

$message = '------=_Part_18243133_1346573420.1408991447668
Content-Type: text/plain; charset=UTF-8

Hello world! This is plain text...


------=_Part_18243133_1346573420.1408991447668
Content-Type: text/html; charset=UTF-8

<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
</head>
<body>

<p>Hello World! This is HTML...</p> 

</body>
</html>


------=_Part_18243133_1346573420.1408991447668--';

$headers = "MIME-Version: 1.0\r\n";
$headers .= "From: Foo <foo@bar.com>\r\n";
$headers .= 'Content-Type: multipart/alternative;boundary="----=_Part_18243133_1346573420.1408991447668"';


// send email
wp_mail( $to, $subject, $message, $headers );

And if you want to change the default content type, use:

add_filter( 'wp_mail_content_type', 'set_content_type' );
function set_content_type( $content_type ) {
    return 'multipart/alternative';
}

This will send a multipart message.

So if you check the full raw source of the message, you'll notice that the content type is added twice, once without boundary:

MIME-Version: 1.0
Content-Type: multipart/alternative;
         boundary="====f230673f9d7c359a81ffebccb88e5d61=="
MIME-Version: 1.0
Content-Type: multipart/alternative; charset=

That's the issue.

The source of the problem lies in pluggable.php - if we look somewhere here:

// Set Content-Type and charset
    // If we don't have a content-type from the input headers
    if ( !isset( $content_type ) )
        $content_type = 'text/plain';

    /**
     * Filter the wp_mail() content type.
     *
     * @since 2.3.0
     *
     * @param string $content_type Default wp_mail() content type.
     */
    $content_type = apply_filters( 'wp_mail_content_type', $content_type );

    $phpmailer->ContentType = $content_type;

    // Set whether it's plaintext, depending on $content_type
    if ( 'text/html' == $content_type )
        $phpmailer->IsHTML( true );

    // If we don't have a charset from the input headers
    if ( !isset( $charset ) )
        $charset = get_bloginfo( 'charset' );

    // Set the content-type and charset

    /**
     * Filter the default wp_mail() charset.
     *
     * @since 2.3.0
     *
     * @param string $charset Default email charset.
     */
    $phpmailer->CharSet = apply_filters( 'wp_mail_charset', $charset );

    // Set custom headers
    if ( !empty( $headers ) ) {
        foreach( (array) $headers as $name => $content ) {
            $phpmailer->AddCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );
        }

        if ( false !== stripos( $content_type, 'multipart' ) && ! empty($boundary) )
            $phpmailer->AddCustomHeader( sprintf( "Content-Type: %s;\n\t boundary=\"%s\"", $content_type, $boundary ) );
    }

    if ( !empty( $attachments ) ) {
        foreach ( $attachments as $attachment ) {
            try {
                $phpmailer->AddAttachment($attachment);
            } catch ( phpmailerException $e ) {
                continue;
            }
        }
    }

Potential solutions

So you are wondering, why have you not reported this at trac? I already have. To my great surprise, a different ticket was created 5 years ago outlining the same problem.

Let's face it, it's been a half decade. In internet years, that is more like 30. The issue has clearly been abandoned and basically will never be fixed (...unless if we resolve it here).

I found a great thread here offering a solution, but while his solution works, it breaks emails that do not have custom $headers set.

That's where we crash every time. Either the multipart version work fine, and normal unset $headers messages don't, or vise verse.

The solution we came up with was:

if ( false !== stripos( $content_type, 'multipart' ) && ! empty($boundary) ) {
    $phpmailer->ContentType = $content_type . "; boundary=" . $boundary;
}
else {

        $content_type = apply_filters( 'wp_mail_content_type', $content_type );

    $phpmailer->ContentType = $content_type;

    // Set whether it's plaintext, depending on $content_type
    if ( 'text/html' == $content_type )
        $phpmailer->IsHTML( true );

    // If we don't have a charset from the input headers
    if ( !isset( $charset ) )
        $charset = get_bloginfo( 'charset' );
}

// Set the content-type and charset

/**
 * Filter the default wp_mail() charset.
 *
 * @since 2.3.0
 *
 * @param string $charset Default email charset.
 */
$phpmailer->CharSet = apply_filters( 'wp_mail_charset', $charset );

// Set custom headers
if ( !empty( $headers ) ) {
    foreach( (array) $headers as $name => $content ) {
        $phpmailer->AddCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );
    }

}

Yes, I know, editing core files are taboo, sit back down... this was a desperate fix and a poor attempt to provide a fix for core.

The problem with our fix is that default emails like new registrations, comment, password reset etc will be delivered as blank messages. So we have a working wp_mail() script that will send multipart messages but nothing else.

What to do

The aim here is to find a way to send both normal (plain text) and multipart messages using the core wp_mail() function (not a custom sendmail function).

When attempting to solve this, the main problem you will encounter is the amount of time that you'll spend on sending dummy messages, checking if they're received and basically opening a box of aspirin and cursing at Microsoft because you are used to their IE issues while the gremlin here is unfortunately WordPress.

Update

The solution posted by @bonger allows $message to be an array containing content-type keyed alternates. I have confirmed that it works in all scenarios.

We will allow this question to remain open until bounty runs out to raise awareness about the problem, maybe to a level where it will be fixed in core. Feel free to post an alternative solution where $message can be a string.

  • 1
    As the wp_mail() function is pluggable is not defining your replacement as a must-use plugin (in wp-content/mu-plugins) not a good solution for you (and everyone else, failing core fix)? In which case would not moving the multipart/boundary check to after setting $phpmailer->ContentType = $content_type; (rather than elsing) not work? – bonger Jun 18 '15 at 18:35
  • @bonger Can you please write up an answer detailing your solution? – Christine Cooper Jun 18 '15 at 19:07
  • 1
    Yo don't need to edit core, because the wp_mail is pluggable. Copy original function in a plugin, edit it like you need and activate the plugin. WordPress will use your edited function instead of the original one, with no need to edit core. – gmazzap Jun 19 '15 at 1:41
  • @ChristineCooper I hesitate to do this as as you say testing is such a royal pain, but looking at the patch core.trac.wordpress.org/ticket/15448 suggested in trac by @rmccue/@MattyRob that looks a really nice way to go so I'll post an untested answer based on that... – bonger Jun 19 '15 at 5:17
  • 2
    @ChristineCooper if you simple hook into phpmailer and set the text body in $phpmailer->AltBody the same error happens? – chifliiiii Jul 10 '15 at 16:52
14
+100

The following version of wp_mail() is with the patch applied of @rmccue/@MattyRob in the ticket https://core.trac.wordpress.org/ticket/15448, refreshed for 4.2.2, which allows $message to be an array containing content-type keyed alternates:

/**
 * Send mail, similar to PHP's mail
 *
 * A true return value does not automatically mean that the user received the
 * email successfully. It just only means that the method used was able to
 * process the request without any errors.
 *
 * Using the two 'wp_mail_from' and 'wp_mail_from_name' hooks allow from
 * creating a from address like 'Name <email@address.com>' when both are set. If
 * just 'wp_mail_from' is set, then just the email address will be used with no
 * name.
 *
 * The default content type is 'text/plain' which does not allow using HTML.
 * However, you can set the content type of the email by using the
 * 'wp_mail_content_type' filter.
 *
 * If $message is an array, the key of each is used to add as an attachment
 * with the value used as the body. The 'text/plain' element is used as the
 * text version of the body, with the 'text/html' element used as the HTML
 * version of the body. All other types are added as attachments.
 *
 * The default charset is based on the charset used on the blog. The charset can
 * be set using the 'wp_mail_charset' filter.
 *
 * @since 1.2.1
 *
 * @uses PHPMailer
 *
 * @param string|array $to Array or comma-separated list of email addresses to send message.
 * @param string $subject Email subject
 * @param string|array $message Message contents
 * @param string|array $headers Optional. Additional headers.
 * @param string|array $attachments Optional. Files to attach.
 * @return bool Whether the email contents were sent successfully.
 */
function wp_mail( $to, $subject, $message, $headers = '', $attachments = array() ) {
    // Compact the input, apply the filters, and extract them back out

    /**
     * Filter the wp_mail() arguments.
     *
     * @since 2.2.0
     *
     * @param array $args A compacted array of wp_mail() arguments, including the "to" email,
     *                    subject, message, headers, and attachments values.
     */
    $atts = apply_filters( 'wp_mail', compact( 'to', 'subject', 'message', 'headers', 'attachments' ) );

    if ( isset( $atts['to'] ) ) {
        $to = $atts['to'];
    }

    if ( isset( $atts['subject'] ) ) {
        $subject = $atts['subject'];
    }

    if ( isset( $atts['message'] ) ) {
        $message = $atts['message'];
    }

    if ( isset( $atts['headers'] ) ) {
        $headers = $atts['headers'];
    }

    if ( isset( $atts['attachments'] ) ) {
        $attachments = $atts['attachments'];
    }

    if ( ! is_array( $attachments ) ) {
        $attachments = explode( "\n", str_replace( "\r\n", "\n", $attachments ) );
    }
    global $phpmailer;

    // (Re)create it, if it's gone missing
    if ( ! ( $phpmailer instanceof PHPMailer ) ) {
        require_once ABSPATH . WPINC . '/class-phpmailer.php';
        require_once ABSPATH . WPINC . '/class-smtp.php';
        $phpmailer = new PHPMailer( true );
    }

    // Headers
    if ( empty( $headers ) ) {
        $headers = array();
    } else {
        if ( !is_array( $headers ) ) {
            // Explode the headers out, so this function can take both
            // string headers and an array of headers.
            $tempheaders = explode( "\n", str_replace( "\r\n", "\n", $headers ) );
        } else {
            $tempheaders = $headers;
        }
        $headers = array();
        $cc = array();
        $bcc = array();

        // If it's actually got contents
        if ( !empty( $tempheaders ) ) {
            // Iterate through the raw headers
            foreach ( (array) $tempheaders as $header ) {
                if ( strpos($header, ':') === false ) {
                    if ( false !== stripos( $header, 'boundary=' ) ) {
                        $parts = preg_split('/boundary=/i', trim( $header ) );
                        $boundary = trim( str_replace( array( "'", '"' ), '', $parts[1] ) );
                    }
                    continue;
                }
                // Explode them out
                list( $name, $content ) = explode( ':', trim( $header ), 2 );

                // Cleanup crew
                $name    = trim( $name    );
                $content = trim( $content );

                switch ( strtolower( $name ) ) {
                    // Mainly for legacy -- process a From: header if it's there
                    case 'from':
                        $bracket_pos = strpos( $content, '<' );
                        if ( $bracket_pos !== false ) {
                            // Text before the bracketed email is the "From" name.
                            if ( $bracket_pos > 0 ) {
                                $from_name = substr( $content, 0, $bracket_pos - 1 );
                                $from_name = str_replace( '"', '', $from_name );
                                $from_name = trim( $from_name );
                            }

                            $from_email = substr( $content, $bracket_pos + 1 );
                            $from_email = str_replace( '>', '', $from_email );
                            $from_email = trim( $from_email );

                        // Avoid setting an empty $from_email.
                        } elseif ( '' !== trim( $content ) ) {
                            $from_email = trim( $content );
                        }
                        break;
                    case 'content-type':
                        if ( is_array($message) ) {
                            // Multipart email, ignore the content-type header
                            break;
                        }
                        if ( strpos( $content, ';' ) !== false ) {
                            list( $type, $charset_content ) = explode( ';', $content );
                            $content_type = trim( $type );
                            if ( false !== stripos( $charset_content, 'charset=' ) ) {
                                $charset = trim( str_replace( array( 'charset=', '"' ), '', $charset_content ) );
                            } elseif ( false !== stripos( $charset_content, 'boundary=' ) ) {
                                $boundary = trim( str_replace( array( 'BOUNDARY=', 'boundary=', '"' ), '', $charset_content ) );
                                $charset = '';
                            }

                        // Avoid setting an empty $content_type.
                        } elseif ( '' !== trim( $content ) ) {
                            $content_type = trim( $content );
                        }
                        break;
                    case 'cc':
                        $cc = array_merge( (array) $cc, explode( ',', $content ) );
                        break;
                    case 'bcc':
                        $bcc = array_merge( (array) $bcc, explode( ',', $content ) );
                        break;
                    default:
                        // Add it to our grand headers array
                        $headers[trim( $name )] = trim( $content );
                        break;
                }
            }
        }
    }

    // Empty out the values that may be set
    $phpmailer->ClearAllRecipients();
    $phpmailer->ClearAttachments();
    $phpmailer->ClearCustomHeaders();
    $phpmailer->ClearReplyTos();

    $phpmailer->Body= '';
    $phpmailer->AltBody= '';

    // From email and name
    // If we don't have a name from the input headers
    if ( !isset( $from_name ) )
        $from_name = 'WordPress';

    /* If we don't have an email from the input headers default to wordpress@$sitename
     * Some hosts will block outgoing mail from this address if it doesn't exist but
     * there's no easy alternative. Defaulting to admin_email might appear to be another
     * option but some hosts may refuse to relay mail from an unknown domain. See
     * https://core.trac.wordpress.org/ticket/5007.
     */

    if ( !isset( $from_email ) ) {
        // Get the site domain and get rid of www.
        $sitename = strtolower( $_SERVER['SERVER_NAME'] );
        if ( substr( $sitename, 0, 4 ) == 'www.' ) {
            $sitename = substr( $sitename, 4 );
        }

        $from_email = 'wordpress@' . $sitename;
    }

    /**
     * Filter the email address to send from.
     *
     * @since 2.2.0
     *
     * @param string $from_email Email address to send from.
     */
    $phpmailer->From = apply_filters( 'wp_mail_from', $from_email );

    /**
     * Filter the name to associate with the "from" email address.
     *
     * @since 2.3.0
     *
     * @param string $from_name Name associated with the "from" email address.
     */
    $phpmailer->FromName = apply_filters( 'wp_mail_from_name', $from_name );

    // Set destination addresses
    if ( !is_array( $to ) )
        $to = explode( ',', $to );

    foreach ( (array) $to as $recipient ) {
        try {
            // Break $recipient into name and address parts if in the format "Foo <bar@baz.com>"
            $recipient_name = '';
            if( preg_match( '/(.*)<(.+)>/', $recipient, $matches ) ) {
                if ( count( $matches ) == 3 ) {
                    $recipient_name = $matches[1];
                    $recipient = $matches[2];
                }
            }
            $phpmailer->AddAddress( $recipient, $recipient_name);
        } catch ( phpmailerException $e ) {
            continue;
        }
    }

    // If we don't have a charset from the input headers
    if ( !isset( $charset ) )
        $charset = get_bloginfo( 'charset' );

    // Set the content-type and charset

    /**
     * Filter the default wp_mail() charset.
     *
     * @since 2.3.0
     *
     * @param string $charset Default email charset.
     */
    $phpmailer->CharSet = apply_filters( 'wp_mail_charset', $charset );

    // Set mail's subject and body
    $phpmailer->Subject = $subject;

    if ( is_string($message) ) {
        $phpmailer->Body = $message;

        // Set Content-Type and charset
        // If we don't have a content-type from the input headers
        if ( !isset( $content_type ) )
            $content_type = 'text/plain';

        /**
         * Filter the wp_mail() content type.
         *
         * @since 2.3.0
         *
         * @param string $content_type Default wp_mail() content type.
         */
        $content_type = apply_filters( 'wp_mail_content_type', $content_type );

        $phpmailer->ContentType = $content_type;

        // Set whether it's plaintext, depending on $content_type
        if ( 'text/html' == $content_type )
            $phpmailer->IsHTML( true );

        // For backwards compatibility, new multipart emails should use
        // the array style $message. This never really worked well anyway
        if ( false !== stripos( $content_type, 'multipart' ) && ! empty($boundary) )
            $phpmailer->AddCustomHeader( sprintf( "Content-Type: %s;\n\t boundary=\"%s\"", $content_type, $boundary ) );
    }
    elseif ( is_array($message) ) {
        foreach ($message as $type => $bodies) {
            foreach ((array) $bodies as $body) {
                if ($type === 'text/html') {
                    $phpmailer->Body = $body;
                }
                elseif ($type === 'text/plain') {
                    $phpmailer->AltBody = $body;
                }
                else {
                    $phpmailer->AddAttachment($body, '', 'base64', $type);
                }
            }
        }
    }

    // Add any CC and BCC recipients
    if ( !empty( $cc ) ) {
        foreach ( (array) $cc as $recipient ) {
            try {
                // Break $recipient into name and address parts if in the format "Foo <bar@baz.com>"
                $recipient_name = '';
                if( preg_match( '/(.*)<(.+)>/', $recipient, $matches ) ) {
                    if ( count( $matches ) == 3 ) {
                        $recipient_name = $matches[1];
                        $recipient = $matches[2];
                    }
                }
                $phpmailer->AddCc( $recipient, $recipient_name );
            } catch ( phpmailerException $e ) {
                continue;
            }
        }
    }

    if ( !empty( $bcc ) ) {
        foreach ( (array) $bcc as $recipient) {
            try {
                // Break $recipient into name and address parts if in the format "Foo <bar@baz.com>"
                $recipient_name = '';
                if( preg_match( '/(.*)<(.+)>/', $recipient, $matches ) ) {
                    if ( count( $matches ) == 3 ) {
                        $recipient_name = $matches[1];
                        $recipient = $matches[2];
                    }
                }
                $phpmailer->AddBcc( $recipient, $recipient_name );
            } catch ( phpmailerException $e ) {
                continue;
            }
        }
    }

    // Set to use PHP's mail()
    $phpmailer->IsMail();

    // Set custom headers
    if ( !empty( $headers ) ) {
        foreach ( (array) $headers as $name => $content ) {
            $phpmailer->AddCustomHeader( sprintf( '%1$s: %2$s', $name, $content ) );
        }
    }

    if ( !empty( $attachments ) ) {
        foreach ( $attachments as $attachment ) {
            try {
                $phpmailer->AddAttachment($attachment);
            } catch ( phpmailerException $e ) {
                continue;
            }
        }
    }

    /**
     * Fires after PHPMailer is initialized.
     *
     * @since 2.2.0
     *
     * @param PHPMailer &$phpmailer The PHPMailer instance, passed by reference.
     */
    do_action_ref_array( 'phpmailer_init', array( &$phpmailer ) );

    // Send!
    try {
        return $phpmailer->Send();
    } catch ( phpmailerException $e ) {
        return false;
    }
}

So if you put that in your eg "wp-content/mu-plugins/functions.php" file then it will override the WP version. It has a nice usage without any messing around with headers, eg:

// Set $to to an hotmail.com or outlook.com email
$to = "YourEmail@hotmail.com";

$subject = 'wp_mail testing multipart';

$message['text/plain'] = 'Hello world! This is plain text...';
$message['text/html'] = '<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
</head>
<body>

<p>Hello World! This is HTML...</p> 

</body>
</html>';

add_filter( 'wp_mail_from', $from_func = function ( $from_email ) { return 'foo@bar.com'; } );
add_filter( 'wp_mail_from_name', $from_name_func = function ( $from_name ) { return 'Foo'; } );

// send email
wp_mail( $to, $subject, $message );

remove_filter( 'wp_mail_from', $from_func );
remove_filter( 'wp_mail_from_name', $from_name_func );

Please note I haven't tested this with actual emails...

  • I have added this to must have plugins and ran the test code; it worked. I have tested default core notifications (new user notification etc) and it worked too. I will continue to perform tests this weekend and see how plugins will work with this and basically if everything works. I will specifically look through the raw data of the message. This will be a very time consuming task but rest assured, I will report back when done. If there is a scenario where wp_mail() will not work (when it otherwise should), please let me know. Thank you for this answer. – Christine Cooper Jun 20 '15 at 15:43
  • Good stuff, I've eye-balled the output and it looks good - in fact the patch just makes wp_mail use PHPMailer's standard rock solid processing in the case of passing an array, and otherwise defaults to the dodgy WP stuff (for backward compatibility) so it should be good (obviously kudos here goes to the patch authors) ... I'm going to be using it from now on (and retro-fitting eventually) - and thanks back for the info re using both html/plain to reduce the chances of being tarred as spam... – bonger Jun 20 '15 at 21:02
  • 1
    We've tested it in all possible scenarios and it's working great. We will shoot out a newsletter tomorrow and we will see if we receive any complaints from users. The only minor changes we needed to do was to sanitize/unsanitize the array when it's being inserted into the db (have messages in a que in the db where a cron sends it out in small bulks). I will allow this question to remain open and pending until the bounty runs out so that we can bring awareness to this issue. Hopefully, this patch, or an alternative will be added to core. Or more importantly, why not. What are they thinking! – Christine Cooper Jun 21 '15 at 15:53
  • I randomly noticed that you performed an update to the linked trac ticket. Is this an update to this code? If so, could you kindly post this update by editing your answer here as well so that this answer stays uptodate? Thank you very much. – Christine Cooper Sep 8 '15 at 23:49
  • Hi, no it was just a refresh of the patch against the current trunk so that it merges without conflicts (in the hope of it getting some attention), the code is exactly the same... – bonger Sep 9 '15 at 4:05
3

This is not really a WordPress bug at all, it is a phpmailer one in not allowing for custom headers... if you look at class-phpmailer.php:

public function getMailMIME()
{
    $result = '';
    $ismultipart = true;
    switch ($this->message_type) {
        case 'inline':
            $result .= $this->headerLine('Content-Type', 'multipart/related;');
            $result .= $this->textLine("\tboundary=\"" . $this->boundary[1] . '"');
            break;
        case 'attach':
        case 'inline_attach':
        case 'alt_attach':
        case 'alt_inline_attach':
            $result .= $this->headerLine('Content-Type', 'multipart/mixed;');
            $result .= $this->textLine("\tboundary=\"" . $this->boundary[1] . '"');
            break;
        case 'alt':
        case 'alt_inline':
            $result .= $this->headerLine('Content-Type', 'multipart/alternative;');
            $result .= $this->textLine("\tboundary=\"" . $this->boundary[1] . '"');
            break;
        default:
            // Catches case 'plain': and case '':
            $result .= $this->textLine('Content-Type: ' . $this->ContentType . '; charset=' . $this->CharSet);
            $ismultipart = false;
            break;
    }

You can see the offending default case is what is outputting the extra header line with charset and no boundary. Setting the content type by filter does not solve this by itself only because the alt case here is set on message_type by checking AltBody is not empty rather than the content type.

protected function setMessageType()
{
    $type = array();
    if ($this->alternativeExists()) {
        $type[] = 'alt';
    }
    if ($this->inlineImageExists()) {
        $type[] = 'inline';
    }
    if ($this->attachmentExists()) {
        $type[] = 'attach';
    }
    $this->message_type = implode('_', $type);
    if ($this->message_type == '') {
        $this->message_type = 'plain';
    }
}

public function alternativeExists()
{
    return !empty($this->AltBody);
}

In the end what this means is as soon as you attach a file or inline image, or set the AltBody, the offending bug should be bypassed. It also means there is no need to explicitly set the content type because as soon as there is an AltBody it is set to multipart/alternative by phpmailer.

So the simple answer is:

add_action('phpmailer_init','wp_mail_set_text_body');
function wp_mail_set_text_body($phpmailer) {
     if (empty($phpmailer->AltBody)) {$phpmailer->AltBody = strip_tags($phpmailer->Body);}
}

Then you don't need to set the headers explicitly, you can simply do:

 $message ='<html>
 <head>
     <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 </head>
 <body>
     <p>Hello World! This is HTML...</p> 
 </body>
 </html>';

 wp_mail($to,$subject,$message);

Unforunately many of the functions and properties in the phpmailer class are protected, if not for that a valid alternative would be to simply check and override the MIMEHeaders property via the phpmailer_init hook before sending.

2

I just released a plugin to let users use html templates on WordPress and Im playing right now on the dev version to add a simple text fallback. I did the following and in my tests I only see one boundary added and emails are arriving fine to Hotmail.

add_action( 'phpmailer_init', array($this->mailer, 'send_email' ) );

/**
* Modify php mailer body with final email
*
* @since 1.0.0
* @param object $phpmailer
*/
function send_email( $phpmailer ) {

    $message            =  $this->add_template( apply_filters( 'mailtpl/email_content', $phpmailer->Body ) );
    $phpmailer->AltBody =  $this->replace_placeholders( strip_tags($phpmailer->Body) );
    $phpmailer->Body    =  $this->replace_placeholders( $message );
}

So basically what I do in here is modify the phpmailer object , load the message inside a html template and set it to the Body property. Also I took the original message and set the AltBody property.

2

My simply solution is to use html2text https://github.com/soundasleep/html2text in this way:

add_action( 'phpmailer_init', 'phpmailer_init' );

//http://wordpress.stackexchange.com/a/191974
//http://stackoverflow.com/a/2564472
function phpmailer_init( $phpmailer )
{
  if( $phpmailer->ContentType == 'text/html' ) {
    $phpmailer->AltBody = Html2Text\Html2Text::convert( $phpmailer->Body );
  }
}

Here https://gist.github.com/ewake/6c4d22cd856456480bd77b988b5c9e80 also a gist about.

1

this might not be an exact answer to the initial post here, but it's an alternative to some of the solutions here provided regarding setting an alt body

essentially, i needed to (wanted to) set a distinct altbody (i.e plaintext) additionally to the html part instead of relying on some conversion/striptags and whatnot. so i came up with this which seems to work just fine

/* setting the message parts for wp_mail()*/
$markup = array();
$markup['html'] = '<html>some html</html>';
$markup['plaintext'] = 'some plaintext';
/* message we are sending */    
$message = maybe_serialize($markup);


/* setting alt body distinctly */
add_action('phpmailer_init', array($this, 'set_alt_mail_body'));

function set_alt_mail_body($phpmailer){
    if( $phpmailer->ContentType == 'text/html' ) {
        $body_parts = maybe_unserialize($phpmailer->Body);

        if(!empty($body_parts['html'])){
            $phpmailer->MsgHTML($body_parts['html']);
        }

        if(!empty($body_parts['plaintext'])){
            $phpmailer->AltBody = $body_parts['plaintext'];
        }
    }   
}
0

If you do not want to create any code conflict in the Wordpress core, I think the alternative or simplest solution is to add action to phpmailer_init that will do before the actual sending of mail in the wp_mail. To simplified my explanation see the below code example:

<?php 

$to = '';
$subject = '';
$from = '';
$body = 'The text html content, <html>...';

$headers = "FROM: {$from}";

add_action( 'phpmailer_init', function ( $phpmailer ) {
    $phpmailer->AltBody = 'The text plain content of your original text html content.';
} );

wp_mail($to, $subject, $body, $headers);

If you add a content in the PHPMailer class AltBody property then the default content type will automatically set to multipart/alternative.

0

For anyone that's using the 'phpmailer_init' hook to add their own 'AltBody':

The alternative text body is reused for different consecutive mails being sent, unless you clear it manually! WordPress doesn't clear it in wp_mail() because it doesn't expect this property to be used.

This results in recipients potentially receiving mails not meant for them. Luckily most people using HTML-enabled mail clients won’t see the text version, but it’s still basically a security problem.

Luckily there's an easy fix. This includes the altbody replacement bit; note that you do need Html2Text PHP library:

add_filter( 'wp_mail', 'wpse191923_force_phpmailer_reinit_for_multiple_mails', -1 );
function wpse191923_force_phpmailer_reinit_for_multiple_mails( $wp_mail_atts ) {
  global $phpmailer;

  if ( $phpmailer instanceof PHPMailer && $phpmailer->alternativeExists() ) {
    // AltBody property is set, so WordPress must already have used this
    // $phpmailer object just now to send mail, so let's
    // clear the AltBody property
    $phpmailer->AltBody = '';
  }

  // Return untouched atts
  return $wp_mail_atts;
}

add_action( 'phpmailer_init', 'wpse191923_phpmailer_init_altbody', 1000, 1 );
function wpse191923_phpmailer_init_altbody( $phpmailer ) {
  if ( ( $phpmailer->ContentType == 'text/html' ) && empty( $phpmailer->AltBody ) ) {
    if ( ! class_exists( 'Html2Text\Html2Text' ) ) {
      require_once( 'Html2Text.php' );
    }
    if ( ! class_exists( 'Html2Text\Html2TextException' ) ) {
      require_once( 'Html2TextException.php' );
    }
    $phpmailer->AltBody = Html2Text\Html2Text::convert( $phpmailer->Body );
  }
}

Here's also a gist for a WP plugin I modified to fix this problem: https://gist.github.com/youri--/c4618740b7c50c549314eaebc9f78661

Unfortunately I can't comment on the other solutions using the aforementioned hook, to warn them of this, as I don't have enough rep yet to comment.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.